Does the fan kick in around 90? Should be able to hear it, easy with the windows down.
Worst case short term fix is to have the windows open, angle the vents outwards and have the heater blowers on full, helps knock a few degrees off and might avert a HG failure until you can discover the real prob.
Whats the condition of the radiator? The older black steel ones get rusty & crusty. Maybe one too many cooling fins have been dented/gummed up.
Oh check your water for signs of oil film and check under your oil cap for white mayonase deposits. Would indicate you already have a HG failure causing the overheat in which case it's more work.
